Don't forget that most humanoids in the ST universe owe their existance to the "first ones" from the TNG episode "The Chase".
And let's not forget that humans are definitely among those - their DNA samples were part of the code that allowed our heroes to decipher the message of the Ancestors. So differences
between humans and certain aliens can't be categorically explained by referring to those Ancestors and their Great Plan or whatever, even if similarities
Humans seem to be an exception in this maturing business, reaching "teen actor" appearance only in their teens rather than in their preteens. But Molly O'Brien might be a slight exception to the exception...
Perhaps modern medicine not only makes old people healthier and more youthful, but also makes young people healthier and more mature?